Beam splitting ball lens method for its manufacture and apparatus for its packaging

ABSTRACT

Accordingly, a beam-splitting ball lens is provided. The beam-splitting ball lens has: a ball lens; and a beam-splitter filter disposed within the ball lens. The ball lens preferably has first and second portions wherein the beam-splitter filter is disposed at a junction between the first and second portions. The beam-splitting ball lens can further have a mid-plane optical element disposed at the junction such as, a wavelength selective filter, a polarization component, an amplitude modulation mask, a phase modulation mask, a hologram and/or a grating. Also provided is a method for fabricating the beam-splitting ball lens of the present invention. The method includes the steps of: providing the ball lens; and disposing the beam-splitter filter within the ball lens. Preferably the disposing step includes: dividing the ball lens into first and second portions; and disposing the beam-splitter filter at the junction between the first and second portions. Also provided is a mount for the beam-splitting ball lens of the present invention. The mount has a body, the body having screws to retain the beam-splitting ball lens therein. The body further having access holes for two inputs and two outputs corresponding to the two inputs. The access holes being aligned with the beam-splitter filter such that light inputted to the inputs are directed to corresponding outputs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to beam splitting devices and,more particularly, to a beam splitting ball lens, a method for itsmanufacture, and an apparatus for its packaging.

2. Prior Art

Recent advances of technologies have lead to successful deployments ofoptics to cost-sensitive local area networking environments. To meetrapidly increasing bandwidth requirements for future multimediacomputing and communications, planning for 10 Gb/s Ethernet is alreadyunderway. As technologies become gradually matured, optics will be usedfor even shorter data links, from inter-computer distances tointra-computer distances. Innovative, compact, and cost-effectivepackaging methods of optical components are actively being researched.

FIG. 1(a) illustrates a conventional beam splitter 106 and method toperform image relay and split. The beam-splitter 106 is shown surroundedby two pairs of imaging lenses 102, 104. Out of the four possible ports108, 110, 112, 114 are two-input 108, 114 and two-output 110, 112 portswhere input and split/combined output images are located. Such a systemhas been used to facilitate optical branching functions for parallelarray of FIG. 1(a) is bulky, not to mention potential alignment andpackaging complexities resulting therefrom. Thus, conventionalbeam-splitters 106 do not lend themselves to packaging methods ofoptical components that will be needed for future multimedia computingand communications.

SUMMARY OF THE INVENTION

Therefore it is an object of the present invention to provide abeam-splitting ball lens which is more compact than conventionalbeam-splitters.

It is a further object of the present invention to provide abeam-splitting ball lens which is more easily aligned with other opticalcomponents than conventional beam-splitters.

It is still yet a further object of the present invention to provide abeam-splitting ball lens which is packaged more easily than conventionalbeam-splitters.

Accordingly, a beam-splitting ball lens is provided. The beam-splittingball lens comprises: a ball lens; and a beam-splitter filter disposedwithin the ball lens. The ball lens preferably comprises first andsecond portions wherein the beam-splitter filter is disposed at ajunction between the first and second portions. The beam-splitting balllens can further comprise a mid-plane optical element disposed at thejunction, such as, a wavelength selective filter, a polarizationcomponent, an amplitude modulation mask, a phase modulation mask, ahologram and/or a grating.

Also provided is a method for fabricating the beam-splitting ball lensof the present invention. The method comprises the steps of: providing aball lens; and disposing a beam-splitter filter within the ball lens.Preferably the disposing step comprises: dividing the ball lens intofirst and second portions; and disposing the beam-splitter filter at ajunction between the first and second portions.

Also provided is a mount for the beam-splitting ball lens of the presentinvention. The mount comprises a body, the body having a means to retainthe beam-splitting ball lens therein. The body further having accessholes for two inputs and two outputs corresponding to the two inputs.The access holes being aligned with the beam-splitter filter such thatlight inputted to the inputs are directed to corresponding outputs.

Also provided is an add/drop multiplexer for downloading information ofa predetermined wavelength from a plurality of wavelengths. The add/dropmultiplexer comprises: a ball lens; a wavelength filter disposed withinthe ball lens for transmitting the predetermined wavelength, andreflecting the plurality of wavelengths except for the predeterminedwavelength; an input port for transmitting the plurality of wavelengthsto the ball lens; an output port for transmitting the plurality ofwavelengths from the ball lens; a drop port for transmitting thetransmitted predetermined wavelength from the ball lens; and an add portfor adding the predetermined wavelength to the reflected wavelengths.Wherein the input, output, drop, and add ports are arranged about theball lens such that the reflected wavelengths are transmitted to theoutput port, the transmitted wavelength is transmitted to the drop port,and the added predetermined wavelength is transmitted to the output port

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the apparatus andmethods of the present invention will become better understood withregard to the following description, appended claims, and accompanyingdrawings where:

FIG. 1(a) illustrates an image relay/split unit using four lenses and aconventional beam-splitter.

FIG. 1(b) illustrates a beam-splitting ball lens of the presentinvention delivering the same functionality as the image relay/splitunit of FIG. 1(a).

FIG. 2 illustrates a ray tracing geometry of a conventional ball lens.

FIG. 3(a) is a graph illustrating a normalized lateral image error Δ/fvs. normalized output distance X/f for the beam-splitting ball lens ofthe present invention wherein dotted lines illustrate different inputangles a and the solid line illustrates a caustic function indicating anoptimum Δ/f.

FIG. 3(b) is a graph illustrating trade-off relations between excesspower loss (in dB) and the obtainable resolution (in lp/nm) with giveninput NA.

FIG. 4 is a graph illustrating typical power splitting data oftransmitted and reflective beams vs. incident beam angle for a λ=650 mmbeam splitting ball lens.

FIGS. 5(a) and 5(b) illustrate two alternative versions of thebeam-splitting ball lens of the present invention wherein one of thehalves of the ball lens is shaped to accommodate a mid-plane opticalelement.

FIG. 6(a) is an isometric view illustrating a mount for packaging fiberimage guides and a beam-splitting ball lens of the present inventionwherein the mount is configured as a 4-way image splitter/combiner.

FIG. 6(b) is a sectional view illustrating the mount of FIG. 6(a) takenabout line 6 b—6 b.

FIG. 6(c) is a photograph of the mount of FIG. 6(a).

FIG. 6(d) illustrates a typical output image of group 3 elements of atarget.

FIG. 7 illustrates a WDM add/drop multiplexer using a variation of thebeam-splitting ball lens of the present invention.

FIG. 8 illustrates the beam-splitting ball lens of the present inventionused with image fibers having curved ends to minimize off-axisaberrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention introduces a new integrated optical component, abeam-splitting ball lens, which integrates the functionality of fivediscrete optical components, namely four imaging lenses and abeam-splitter, into a single compact component. The integratedbeam-splitting ball lens is a useful functional component for parallelchannel optical circuitry to handle future short-distance opticalinterconnection needs.

Referring now to FIG. 1(b) there is illustrated a beam-splitting ballsystem 200 having a beam-splitting ball lens 202 which replaces thefunctionality of the setup shown in FIG. 1(a). The beam-splitting balllens 202 can be formed by cutting a conventional ball lens into twoportions, preferably halves 204, 206, forming a beam-splitter filter 208at a mid-plane junction 210 between the halves 204, 206. However, thebeam-splitting ball lens 202 may also be integrally formed with thebeam-splitter filter 208, such as by insert injection molding of theball lens with the beam-splitter filter 208 inserted in the mold. Thebeam-splitter filter 208 may be a membrane or coating or other types ofbeam-splitters known in the art. The ball lens may be glass, crystal,semiconductor, or a polymer. The halves 204, 206 are preferably the samesize and shape such that the mid-plane junction 210 in which thebeam-splitter filter 208 is formed or disposed in the middle of the balllens. However, the beam-splitter filter 208 could be offset from themiddle of the ball lens without departing from the scope or spirit ofthe present invention. Preferably, the two halves 204, 206 with thebeam-splitter filter 208 are cemented or adhered together back into aball shape with a suitable optical epoxy. However, the components do nothave to be adhered to one another, they can also be retained in a ballshape by other means, such as the packaging mount discussed below.

The division of the ball lens does not have to be by time-consumingcutting or polishing, but may be done by directly forming half ballsusing a polymer injection molding process or a casting process. The balllens halves may also be formed by conventional grinding. Once thebeam-splitting ball lens 202 is formed, it can be used in the FIG. 1(a)geometry to deliver 2×2 image splitting/combining operations. Inaddition, the beam-splitting ball lens 202 has a larger angular usagerange than the setup of FIG. 1(a) which is limited by the placements ofthe four discrete lenses. Although the beam splitting ball lens 202 isshown with the beam-splitter filter 208 oriented 45° relative to thepath of input light, its orientation can be at any angle. Alternatively,the beam-splitting ball lens 202 can also include a midplane opticalelement (see FIGS. 5a and 5 b) in addition to the beam-splitter filter208. Examples of mid-plane optical elements include a wavelength filter,a polarization filter, or an amplitude/phase mask or grating, to namebut a few. Thus, a new range of functionalities can be incorporatedwhile the basic and essential imaging function is performed.

For imaging, the beam-splitting ball lens 202 has the same feature as asingle ball lens of identical size and material except that it offerstwo output imaging planes. FIG. 2 shows a general ray tracing geometryof a ball lens 300 of radius R and refractive index of n′. For anon-axis ray originated at point A towards point D, the ray enters andexits the ball at B and C before crossing the optic axis at D. For agiven input distance L, A lateral error Δ occurs when the outputdistance is selected at X. An optimum Δ to meet a particular resolutionrequirement can be computed for the ball lens 300. First, from thetriangle ABO and using the sine-law, we have

sin (α)=L/R sin (σ)  (1)

Using Snell's law at the boundary point B and Eq. (1), we have

sin (α′)=nL/n′R sin (α)  (2)

From trigonometry and basic properties of a ball, it can be shown that

σ′=2α−2α′−σ  (3)

Now, the application of the sine-law to triangle OCD yields

L′=R sine (α)/sine (α′)=L sine (θ)/sine (θ′)  (4)

Since the lateral error Δ is defined as

Δ=(X−L′) tan (σ′)  (5)

by substituting Eq. (4) into Eq. (5), we have

Δ_(X,θ)=X [sin(θ′)−sin (θ)]/cos (θ′)  (6)

Using Eq. (1) through (3) and Eq. (6), a complicated expression linkingθ and Δ can be derived. For unity image magnification which is theprimary application of the present invention, θ is maximized for a givenΔ while setting

X=L=2f  (7)

where f is the focal length of the ball lens, using an ideal lensequation derived from paraxial approximations. Since the optimizationprocess does not have an analytic solution, we plot, using dotted ordashed curves, the relations between Δ/f vs. X/f with θ as a changingparameter in FIG. 3(a). The dotted straight line at the bottomcorresponds to θ=0, or paraxial approximation. It can be seen that for anon-zero aperture angle σ, a non-zero lateral error Δ is inevitablygenerated. The envelope function as a solid curve at the top is thecaustic function for the optimum relation between Δ/f and X/f. Theconclusion is that the smaller the input angle to the ball lens 300, thebetter the resolution. However, it is also true that the smaller theinput angle or limiting aperture, the larger the power loss of thesystem. FIG. 3(b) shows some relations between the forced power loss inunit of dB vs. the output resolution in unit of lp/mm with the launchingangle as a parameter.

Based on BK7 ball lenses of different diameters, several beam-splittingball lenses 202 were fabricated by polishing, coating and re-cementing.The beam-splitter filter 208 was a coating designed to have a 50/50power splitting ratio for X=650 nm at a 45° incident angle. As notedbefore, for large volume productions, molding of the ball lens halves ispreferred because the fabrication procedure of the beam splitting balllens 202 can be simplified. The beam-splitting ball lenses 202fabricated had diameters ranging from 2.5 mm to 6 mm. Thesebeam-splitting ball lenses 202 were tested using a specially designedtest system which measures both the power splitting ratios at differentangles and image resolution using an U.S. Air Force (USAF) resolutiontarget, a CCD camera for image acquisition, and a PC for data analysis.For applications to optical interconnections, the target field wasconfined to within a circular area with a radius of 2 mm which is astandard diameter for a 3,500 pixel polymer fiber-image-guide. For aunity magnification case, all tested beam-splitting ball lenses 202 canresolve>30 lp/mm with no apparent geometric distortion when their inputapertures were set to be R/2. It was noticed that this resolution ismuch greater than that the PFIG can supply (around 20/lp). For powermeasurement, it was noticed that most beam-splitting ball lenses 202 candeliver a splitting ratio with a variation of about ±6% from thedesigned 50/50 splitting ratio. FIG. 4 shows measured power splittingratio of the two beams vs. incident angle. There was a 1.5 dB forced orexcess power loss of the system.

Referring now to FIGS. 5(a) and 5(b) there are shown alternativeversions of the beam-splitting lens of the present invention wherein oneof the halves 204 is substantially half the size of the ball lens andthe other half 206 a, 206 b is smaller by the size of an added mid-planeoptical element 208 a, 208 b. FIG. 5(a) illustrates the beam-splittingball lens 202 of the present invention having both a beam-splitterfilter coating 208 and a filter substrate 208 a. The filter substrate208 a is rectangular in shape, thus, to maintain the ball shape of theball lens, one of the halves 206 a is smaller than the other half 204 bythe size of the rectangular filter substrate 208 a. FIG. 5(b)illustrates a similar embodiment, however, the filter substrate 208 b iswedge shaped and the other half 206 b is shaped to accommodate the wedgeshaped filter substrate 208 b. Of course, other shaped mid-plane opticalelements are possible.

Referring now to FIGS. 6(a) and 6(b), to facilitate packaging of thebeam-splitting ball lens 202, a mount 600 is provided which helps tocouple light between the four input/output optical fiber image guides602, 604, 606, 608 and a 4 mm diameter beam-splitting ball lens 202. Themount has a body 610, preferably fabricated from aluminum, having ameans to retain the beam-splitting ball lens 202 therein. The bodyfurther having access holes 602 a, 604 a, 606 a, 608 a for the opticalfiber image guides 602, 604, 606, 608. The access holes 602 a, 604 a,606 a, 608 a are aligned with the beam-splitter filter 208 such thatlight inputted to the inputs 602, 608 are directed to correspondingoutputs 604, 606.

The body 610 preferably comprises a unitary block having a threadedthrough hole 611 for housing the beam-splitting ball lens 202 within themount 600. The beam-splitting ball lens 202 is retained in the threadedthrough hole 611 by means of a screw plug 612 threadingly engaged onboth sides of the threaded through hole 611. Each screw plug 612 has athreaded portion 614 which mates with a corresponding threaded portion616 of the threaded through hole 611. Each screw plug 612 preferably hasa concavity 618 corresponding to the outer surface of the beam-splittingball lens 202. The vertical positioning of the beam-splitting ball lens202 along arrow A is accomplished by advancing one of the screw plugs612 while withdrawing the other an equal amount.

The mount 600 further has means to fix and adjust the optical fiberimage guides 602, 604, 606, 608 in the access holes 602 a, 604 a, 606 a,608 a of the body 610. This means preferably comprises a threadedbushing 620 threadingly fixed in each of the access holes 602 a, 604 a,606 a, 608 a and having a bore 620 for passage of a correspondingoptical fiber image guide 602, 604, 606, 608. A cap 622 having aninternal threaded portion 622 a which threadingly mates with a portionof the threaded bushing protruding from the body 610. The cap 622further has a bore 622 b axially aligned with the bore 620 a of thethreaded bushing 620. Each optical fiber image guide 602, 604, 606, 608is passed through the bores 622b, 620 a of the cap 622 and threadedbushing 620, respectively, and is retained therein by an o-ring 624which is squeezed around the outer periphery of the optical fiber imageguide when the cap 622 is advanced over the threaded bushing 620.

FIG. 6(c) shows a photograph of a packaged four-way optical imagesplitter/combiner (mount) 600 which uses a combination of bothguided-wave components, i.e. optical fiber image guides, and afree-space component, i.e. a beam-splitting ball lens 202. The assemblyhas a 1.5 dB excess and a power splitting ratio of T/R=0.3 dB.

FIG. 6(d) shows a typical image (group 3) of the USAF target at anoutput port of the packaged four-way image combiner/splitter. Thus, onlyabout 11-12 lp/mm can be resolved, primarily due to a resolution limitof the optical fiber image guides (about 20 lp/mm) and an effect ofcascading two optical fiber image guides and a lens. Nevertheless, theresolution is sufficient to resolve a 2D laser pattern with a laserpitched at 125 μm.

Referring now to FIG. 7, there is illustrated a WDM add/drop multiplexer(ADM), referred to generally by reference numeral 700 utilizing avariation of the beam-splitting ball lens 202 of the present invention,designated by reference numeral 701. The ADM ball lens 701 has awavelength filter 702 disposed at the junction 210 between its twohalves 204, 206 instead of the beam-splitting filter 208. The WDMadd/drop multiplexer 700 is a 4-port device with a main input port 704,a main output port 706, a drop channel output port 708, and an addchannel input port 710. The ports are typically optical fibers. ADM'sare useful to download information associated with a particularwavelength channel. The main input port 704 carries all WDM channels ofλ₁, λ₂, . . . . λ_(n), where data for λ_(i) is to be downloaded. Themain output port 706 transmits all channels to somewhere else. Thewavelength filter 702 at the junction 210 of the ball lens halves 204,206 has the functionality that only passes through a designatedwavelength band, for instance λ_(i). The wavelength filter 702 reflectsall other wavelength channels just like a mirror. Thus, when allwavelengths are present at the wavelength filter 702 from the inputchannel 704, only the λ_(i) wavelength passes through the ball lens andis focused into the drop channel 708. All remaining channels arereflected and are focused into the output channel 706. The missingchannel is replaced by a new beam of wavelength λ_(i) sent by the addchannel 710. Since the wavelength filter 702 is designed to pass throughwavelength λ_(i), this wavelength rejoins the remaining wavelengths atthe output port 706 which transmits all wavelengths λ₁, λ₂, . . . .λ_(n)to a trunk line. Thus, with the ADM ball lens 701 of the presentinvention, a single integrated optical component serves as both a filterand a fiber-filter interface device.

Referring now to FIG. 8 there is illustrated the beam-splitting balllens 202 of the present invention packaged with four fiber image guides802, 804, 804, 808. To minimize off-axis aberrations due to the curvedsurface of the beam-splitting ball lens 202, the image fibers 802, 804,804, 808 have curved ends 802 a, 804 a, 806 a, 808 a. Preferably, thecurved ends 802 a, 804 a, 806 a, 808 a each have a radius correspondingto the radius (R) from the center of the beam-splitting ball lens 202.

To summarize, the present invention provides a new integrated opticalcomponent, a beam-splitting ball lens which can serve the need forimaging and splitting 2D data patterns for various data communicationand sensing applications. Also provided is a compact and flexiblepackaging system (mount) to allow the use of a beam-splitting ball lenswith optical fiber image guides which are cost-effective flexible 2Doptical wave-guiding channels. Thus, the beam-splitting ball lens of thepresent invention and its packaging mount will help ease design concernsof future 2D array based large-bandwidth board- and back-plane-leveloptical interconnections.

While there has been shown and described what is considered to bepreferred embodiments of the invention, it will, of course, beunderstood that various modifications and changes in form or detailcould readily be made without departing from the spirit of theinvention. It is therefore intended that the invention be not limited tothe exact forms described and illustrated, but should be constructed tocover all modifications that may fall within the scope of the appendedclaims.

What is claimed is:
 1. A beam-splitting ball lens comprising: a balllens comprising first and second portions; a beam-splitter filterdisposed at a junction between the first and second portions of the balllens, and a mid-plane optical element disposed at the junction betweenthe first and second portions; wherein one of the first or secondportions is substantially half the size of the ball lens and the otherof the first and second portions is smaller than half the size of theball lenses by the size of the mid-plane optical element.
 2. Thebeam-splitting ball lens of claim 1, further comprising an adhesive foradhering the first and second portions and the beam-splitter filtertogether.
 3. The beam-splitting ball lens of claim 1, wherein the firstand second portions are substantially the same size and shape and thebeam-splitter filter is disposed substantially in the middle of thebeam-splitting ball lens.
 4. The beam-splitting ball lens of claim 1,wherein the mid-plane optical element is selected from a groupconsisting of a wavelength selective filter, a polarization component,an amplitude modulation mask, a phase modulation mask, a hologram and/ora grating.
 5. The beam-splitting ball lens of claim 1, wherein themid-plane optical element is rectangular shaped.
 6. The beam-splittingball lens of claim 1, wherein the mid-plane optical element is wedgeshaped.
 7. The beam-splitting ball lens of claim 1, wherein the balllens is fabricated from a material selected from a group consisting ofglass, crystal, semiconductor, and polymer.
 8. A method for fabricatinga beam-splitting ball lens, the method comprising the steps of:providing a ball lens having first and second portions; and disposing abeam-splitter filter at a junction between the first and second portionsof the ball lens; and disposing a mid-plane optical element at thejunction between the first and second portions; wherein one of the firstor second portions is provided substantially half the size of the balllens and the other of the first and second portions is provided smallerthan half the size of the ball lenses by the size of the mid-planeoptical element.
 9. The method of claim 8, wherein the disposing stepcomprises: dividing the ball lens into first and second portions; anddisposing the beam-splitter filter at a junction between the first andsecond portions.
 10. The method of claim 9, further comprising the stepof adhering the first and second portions and the mid-plane opticalelement together with an adhesive.
 11. The method of claim 9, whereinthe dividing step comprises cutting the ball lens into the first andsecond portions.
 12. The method of claim 8, wherein the disposing stepcomprises: individually molding the ball lens into first and secondportions; and disposing the beam splitter filter at a junction betweenthe first and second portions.
 13. The method of claim 8, wherein thedisposing step comprises: individually casting the ball lens into firstand second portions; and disposing the beam splitter filter at ajunction between the first and second portions.
 14. The method of claim8, wherein the providing step comprises fabricating the ball lens from amaterial selected from a group consisting of glass, crystal,semiconductor, and polymer.
 15. A mount for a beam-splitting ball lens,the beam-splitting ball lens comprising a ball lens and a beam-splitterfilter disposed within the ball lens, the mount comprising a body, thebody having a means to retain the beam-splitting ball lens therein, thebody further having access holes for two inputs and two outputscorresponding to the two inputs, the access holes being aligned with thebeam-splitter filter such that light inputted to the inputs are directedto corresponding outputs, the mount further comprising alignment meansfor aligning and adjusting the beam-splitting ball lens relative to theinputted light.
 16. The mount of claim 15, wherein the alignment meanscomprises first and second screw plugs, each screw plug beingthreadingly disposed in a threaded through hole in the body therebysandwiching the beam-splitting ball lens therebetween such thatadvancing one of the screw plugs and withdrawing the other of the screwplugs an equal amount positions the beam-splitting ball lens along avertical axis relative to the inputted light.
 17. The mount of claim 15,wherein light is input to and output from the beam-splitting ball lensby optical fiber image guides, the mount further comprising means to fixand adjust the optical fiber image guides in the access holes of thebody.
 18. The mount of claim 17, wherein the means to fix and adjust theoptical fiber image guides in each of the access holes of the bodycomprises: a threaded bushing disposed in a mating threaded portion ofeach of the access holes, each threaded bushing further having a firstbore for passage of a corresponding optical fiber image guide; a capthreadingly disposed on a portion of each of the threaded bushingsprotruding from the body, each cap further having a second bore axiallyaligned with the first bore for passage of the corresponding opticalfiber image guide; and an o-ring disposed between each threaded bushingand cap such that when the cap is advanced relative to the threadedbushing the o-ring is squeezed around an outer periphery of thecorresponding optical fiber image guide and retained therein.
 19. Amount for a beam-splitting ball lens, the beam-splitting ball lenscomprising a ball lens and a beam-splitter filter disposed within theball lens, the mount comprising a body, the body having a means toretain the beam-splitting ball lens therein, the body further havingaccess holes for two inputs and two outputs corresponding to the twoinputs, the access holes being aligned with the beam-splitter filtersuch that light inputted to the inputs are directed to correspondingoutputs, wherein light is input to and output from the beam-splittingball lens by optical fiber image guides, the mount further comprisingmeans to fix and adjust the optical fiber image guides in the accessholes of the body.
 20. The mount of claim 19, wherein the means to fixand adjust the optical fiber image guides in each of the access holes ofthe body comprises: a threaded bushing disposed in a mating threadedportion of each of the access holes, each threaded bushing furtherhaving a first bore for passage of a corresponding optical fiber imageguide; a cap threadingly disposed on a portion of each of the threadedbushings protruding from the body, each cap further having a second boreaxially aligned with the first bore for passage of the correspondingoptical fiber image guide; and an o-ring disposed between each threadedbushing and cap such that when the cap is advanced relative to thethreaded bushing the o-ring is squeezed around an outer periphery of thecorresponding optical fiber image guide and retained therein.